Bass and Mid Freq drop on JCM900

[Trotter]

I have a Marshall JCM900 (model 4100) which has been my go to amp over the last few years. Its equipped with Sovtech 5881's and I really love it, but recently it seems to have gotten pretty trebly... meaning the settings I've normally used don't sound the same and it seems the treble frequency has really taken over the mid and the bass frequencies. To get it to sound right I now have to turn the treble way down and jack the mid and bass knobs to 8 or more... Could this be tube related?

thanks!

[PRNDL]

>> Could this be tube related?

How old are the Sovtech 5881's? It could also be a preamp tube.
How old are the electrolytics?

There are a few simple tests - tapping the tubes to see if they are microphonic, measuring voltages and checking bias/matching, swapping tubes for known good ones.